The present invention relates to electromagnetic compatibility protection for computers. In particular, the present invention relates to electromagnetic compatibility (EMC) for peripheral devices to computers.
Because personal computers (PCs) include a large number of electrical components, they must meet government limitations for electromagnetic susceptibility and electromagnetic emissions. Susceptibility relates to the effects that external electromagnetic signals have on the computer. Emissions relate to electromagnetic radiation produced by the computer.
To reduce a computer's susceptibility and its emissions, the electronics of most computers are shielded within a metal box. Although this shielding reduces the susceptibility and emissions of the computer's internal electronics, it does not provide protection for peripheral devices that are connected to the electronics of the computer via cables. These peripheral devices include a number of devices that track a user's movements such as keyboards, joysticks, game pads, and mice.
These peripheral devices are typically connected to the computer through four conductors. Two of the conductors carry power between the computer and the peripheral device. The other two conductors carry signals to or from the peripheral device.
The four conductors to the peripheral device have a significant susceptibility to electromagnetic events. In particular, the four conductors are susceptible to electro-fast-transient/burst (EFT/B) events. An EFT/B event is a high frequency spike of one kilovolt or more that occurs on one or more of the computer's three input power lines: power, ground, and neutral. Typically, an EFT/B event causes the voltage of the entire computer and its associated peripheral devices to increase rapidly, relative to earth ground.
The four conductors associated with each peripheral device create a high frequency circuit through their parasitic capacitance to ground. In this context, each conductor acts as one plate of the parasitic capacitance. This parasitic capacitance allows high frequency current to flow through the conductors and/or the input device, causing disruption of proper communication between the peripheral device and the PC. This can also disrupt the internal operation of the peripheral device and/or the PC.
The four conductors are also susceptible to electrostatic discharge (ESD) effects by two means. In the case of a direct discharge at the peripheral device, the conductors provide a path from the device to the PC. In the case of indirect application, where the ESD is applied to a coupling plane near the system, ESD energy is coupled to the four conductors via parasitic capacitance and inductance and then conducted to the ends of the conductors. In addition to susceptibility, current input devices are generally unshielded and emit electromagnetic radiation. Most input devices that track a user's hand or eye movement include electronics, operating at moderately high frequency rates, that can generate electromagnetic radiation. In addition, the cable conductors form a parasitic antenna for unintentional radiation of electromagnetic energy. The energy is sourced from both the desired data signals in the cable (and its harmonics), and the parasitic, and generally higher frequency, electronics in the PC and input device.